The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Various types of climate control systems are known in the art for providing heating, ventilating and/or air conditioning (HVAC). Many of these systems employ one or more air-moving components, including blowers (such as air handlers and circulation fans), condenser fans, draft inducers, etc. These air-moving components are commonly driven by electric motors. While single speed and multi-speed motors are sometimes used to drive air-moving components, discrete speed motors have largely been displaced in recent years by variable speed motors.
Variable speed motors for driving air-moving components in HVAC systems commonly employ square wave excitation and control techniques (sometimes referred to as “6-step” commutation). Typically, such variable speed motors use square wave control signals to control the application of positive and negative do voltages to the motor's three phase windings. At any given time, a positive dc voltage is applied to one of the phase windings, a negative dc voltage is applied to another one of the phase windings, and the third phase winding is unenergized or “open” (the unenergized phase winding is usually not truly left open, but rather “flies” into a catch diode or other device for dissipating residual winding current). By sequentially (and abruptly) rotating the application of positive and negative dc voltages among the three phase windings, a rotating magnetic field is created which causes rotation of the rotor for driving the air-moving component.
FIG. 1 illustrates the phase currents produced in a motor using known square wave commutation techniques (the current offsets are shifted in FIG. 1 to clearly illustrate all three phase currents). Because of the manner in which the phase windings are abruptly switched, with one phase winding unenergized at any given time, the resulting phase currents are discontinuous. As can be seen in FIG. 1, each phase current has a zero voltage level for about one-third of each cycle.
The known square wave commutation techniques and resulting discontinuous phase currents produce relatively high cogging torque, as illustrated in FIG. 2, as well as relatively high operating torque ripple and torque harmonics. This, in turn, produces undesirable acoustic noise and vibration in the motor and thus any HVAC system in which the motor is used. For these reasons, many known HVAC motors couple the rotatable assembly (also referred to as the rotor) to the motor shaft using a mechanical damping material to reduce noise and vibration.
Further, known square wave commutation techniques are considered relatively inefficient, and produce an efficiency loss in the motor on the order of about two percent (2%).